We have identified a novel transmembrane protein tyrosine phosphatase (VPTPalpha) whose expression is highly regulated in rat vascular smooth muscle cells and in endothelial cells. In smooth muscle cells, expression level is correlated with the formation of "epithelioid" monolayers in which adjacent cells neither overlap nor migrate away from each other. We hypothesize that VPTPalpha plays a role in regulating interactions which are involved in the development, maintenance and remodeling of the vessel wall. We will identify and characterize the extracellular "ligand" for VPTPalpha. To obtain insight into the biochemical pathways in which VPTPalpha might function, we will identify cytoplasmic components which interact with the intracellular domain of VPTPalpha (substrates and effectors). We will evaluate the biological function of VPTPalpha by determining its pattern of expression in vivo and by evaluating the effects of experimentally manipulating its level and/or activity in cell culture and in vivo.